Conventionally, there is known a system which is provided with a plurality of control units which control an engine and an automatic gear shift in an automotive vehicle and each of the plurality of control units performs fault detection by itself, respectively. For example, see Japanese Patent No. 2601194.
In the system of Japanese Patent No. 2601194, a control unit having a data communication capability among the plurality of control units is used to transmit data, indicated by the result of the fault detection performed by each control unit, to an external device outside the system.
Moreover, there is also known a system which is provided with a plurality of nodes, and each of the plurality of nodes is provided with a memory which stores data respectively. For example, see Japanese Laid-Open Patent Application No. 10-276198.
A conceivable method for storing the diagnosis data, created by the fault detection performed by the control unit (node) is that each of the plurality of control units is provided with a memory respectively, and the diagnostic data created by one control unit is always stored in its memory of the control unit (self-node storage).
However, with the composition in which the diagnostic data created by one control unit is always stored in its memory of the control unit (self-node storage), the situation where the created diagnostic data cannot be stored in its memory because of lack of the storage capacity of the memory of the control unit concerned or the situation where the previously stored data in the memory of the control unit must be erased in order to store the created diagnostic data may arise.
A conceivable method for avoiding such situations (insufficient storage capacity of the memory) is to provide a memory having a large storage capacity for each of the plurality of control units. However, if such method is used, another problem of the increase in the cost and the loading space will arise.